Complexity and time varying nature of channels are the most obvious characteristics of wireless mobile communications. During a coherent reception, it is necessary to estimate and measure channels at a receiving end, and then perform coherent detection with the obtained channel responses. Besides, the channel estimation is also used in measurement of physical layer, smart antenna, rapid control, handover and management of radio resource etc.
In a time-slotted CDMA cellular mobile communication system, symbols for channel estimation as well as data are transmitted in the Time Division Duplex (TDD) mode while transmissions in different cells are synchronized in the same time-slots.
An example of a burst structure in a TD-SCDMA system is shown in FIG. 1. The middle of burst structure is the midamble, which is used for channel estimation, so it is also called the channel estimation code. Both sides of the midamble are used for transmitting data symbols.
The midamble, i.e. channel estimation code is generated as follows: for the same time-slot of the same cell, one midamble is given as a basic code, and different users take different versions of the same basic code generated by shifting cyclically as their midambles. The K midambles mP(k) generated from the same basic code mP, k=1, . . . K, constitute a set, which is called a midamble set, or briefly, a code-set. In the TD-SCDMA system, the basic code has 128 chips, and length of the generated midamble Lm=128+16 chips.
The channel estimation method for a time-slotted CDMA system is called Steiner Estimator, which is described in the article: B. Steiner and P. W. Baier, “Low Cost Channel Estimation in the Uplink Receiver of CDMA Mobile Radio Systems,” FREQUENZE, 47(1993) 11-12. The specific estimation procedure is described as follows.
Suppose the channel response vector of each user is h(k):
                              h                      (            k            )                          =                              (                                          h                1                                  (                  k                  )                                            ,                              h                2                                  (                  k                  )                                            ,              …              ⁢                                                          ,                              h                W                                  (                  k                  )                                                      )                    T                                    (        1        )            
where W, the window length, represents the time width of the channel response; and the channel response vector h(k) is represented with values of W taps at a chip interval. In this case, the number of taps of the midamble response signal is Lm+W−1. Since the midamble and data symbols are continuously transmitted, the first W−1 tap values of midamble response signal are affected by the data symbols in front, and the last W−1 tap values of the midamble response signal are overlapped by the data symbols behind. Take P chips at the middle of the burst as observed values and the vector is represented as:emid=(e1, e2, . . . , eP)  (2)
Based on the generation characteristics of midambles in a code-set for multiple users, the midamble response signals received at the receiving side is represented as:emid=Gh+n  (3)
where n is noise and interference, and h is total channel response vectors for all users:h=(h(1)T, h(2)T, . . . , h(K)T)T  (4);
G is a circle matrix:G=(circle(g1, g2, . . . , gP))T  (5)
where g=(g1, g2, . . . , gP) is the first column of Matrix G, and is defined by the basic code that is used for generating the code-set.
The task of channel estimation is to solve equation (3) in order to obtain the unknown h. Based on the maximum likelihood method, the channel estimate ĥ can be obtained with the following simplified equation:ĥ=G−1emid  (6)
Since G is a circle matrix, the operation of equation (6) can be made by means of Discrete Fourier transform (DFT) and Inverse Discrete Fourier transform (IDFT):
                              h          ^                =                  IDFT          ⁡                      (                                          DFT                ⁡                                  (                                      e                    mid                                    )                                                            DFT                ⁡                                  (                  g                  )                                                      )                                              (        7        )            
where the DFT (g) can be obtained by offline calculation in advance.
With less computation cost, the channel estimation method mentioned above obtains the channel estimates for multiple users belonging to the same code-set, and suppresses the interference among midambles of multiple users generated from the same basic code. This is a channel estimation method in connection with multi-user midambles in a code set based on one same basic code, that is, a single code-set channel estimation method.
As the single code-set channel estimation method processes signals other than the response signal of the own code-set that are overlapped at the same time period as white Gaussian noise, the method itself is imperfect. Besides, for the time-slotted CDMA systems operating in cell synchronization, signals of adjacent cell users who are located at the boundary of the local cell receive strong interferences from signals of the local cell while signals of adjacent cells transmitted in the same frequency as the signals of the local cell are synchronized with the signals thereof in a time-slot. In other words, the primary interference of the midamble responses code-set of the local cell is caused by the other code sets of the adjacent cells, since they are synchronized at the same time-slot as well as the same frequency. In some other cases, it is possible that midamble responses of multiple code-sets with approximately equal power are overlapped.
For example, in a multiple receiving-antennas situation, the single code-set channel estimation method not only limits the performance of the multiple receiving-antennas system, but also worsens the performance of such subsystem as beam-forming transmission, measurement of physical layer, synchronization and power control. In another example, when there are users of adjacent cells operate at the boundary of the cells in the same frequency, especially in terms of a system with less spread gain, signals of users in adjacent cells cause strong interference to the local cell signals. As a result, the single code-set channel estimation method can not meet the channel estimation requirement in these cases.
On the whole, for the time-slotted CDMA cellular mobile communication system where multiple code-channels of adjacent cells operate in the same frequency, it is necessary to provide a better channel estimation method as the single code-set channel estimation method is often unable to meet the requirements of the system performance.